Two tidal bulges are created on opposite sides of the Earth due to the moon's gravitational force and inertias counterbalance. Where is the world's largest tidal bore? China's Qiantang River has the world's largest tidal bore, which can go up to 30 feet high and 25 miles per hour.
The strongest bores are formed in the fall, making the region a hotspot for surfers from all over the world. Why do we have 2 tides a day? This pair of bulges is the Earth's twin high tides, and they stay put, aligned with the Moon — it is the Earth and ocean rotating beneath them that causes the ocean to rise and fall twice a day in any given place.
It's not just the Moon that pulls on our oceans. The Sun's gravity affects our tides, too. Why does Southampton have two high tides? But the water level at high tide changes too! Why does the moon affect tides but not us? The Moon affects the tides because of gravity. You will have noticed that every time you jump, you always land back on the ground. This is because the Earth's gravity is pulling you back down.
The Moon has gravity of its own, which pulls the oceans and us towards it. Why are there tides? High and low tides are caused by the moon. The moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides. The ocean's tidal bulges are like taking a spherical balloon and squeezing on the sides until the top and bottom bulge out.
Even if the earth stopped spinning, there would still be two tidal bulges. The spinning of earth does lead to centrifugal-force-caused bulging, known as equatorial bulging, but it is a different effect from tidal bulging. Topics: gravitational gradient , gravity , moon , tidal bulge , tides. The white arrows show the direction of the tidal forces due to the gravitational gradient of the moon.
The ocean responds to these forces by bulging up on the near and far side of the earth relative to the moon. At the center of Earth is approximately the average of the moon's gravitational pull on the whole planet. Arrows represent the force of the moon's gravitational pull on Earth. To get the tidal force—the force that causes the tides—we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.
The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges. Arrows represent the tidal force. It's what's left over after removing the moon's average gravitational pull on the whole planet from the moon's specific gravitational pull at each location on Earth. These two bulges explain why in one day there are two high tides and two low tides, as the Earth's surface rotates through each of the bulges once a day. The Sun causes tides just like the moon does, although they are somewhat smaller.
When the earth, moon, and Sun line up—which happens at times of full moon or new moon—the lunar and solar tides reinforce each other, leading to more extreme tides, called spring tides. When lunar and solar tides act against each other, the result is unusually small tides, called neap tides.
There is a new moon or a full moon about every two weeks, so that's how often we see large spring tides. When the gravitational pull of the Sun and moon are combined, you get more extreme high and low tides. This explains high and low tides that happen about every two weeks.
Note: this figure is not to scale. The Sun is much bigger and farther away. Wind and weather patterns also can affect water level. Strong offshore winds can move water away from coastlines, exaggerating low tides. Onshore winds can push water onto the shore, making low tides much less noticeable. High-pressure weather systems can push down sea levels, leading to lower tides. Low-pressure systems—brought on by strong storms and hurricanes—can cause tides than are much higher than predicted, so watch out!
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